1) Subdomains. If you're app utilises subdomains, you need to use a TLD.

2) Integration with development tools like http://pow.cx/ or https://github.com/puma/puma-dev allow you to automatically startup and shutdown apps. This is useful when you're juggling multiple apps.

3) Security. Using a standard domain/port for local development opens you up to exploits like http://benmmurphy.github.io/blog/2016/07/11/rails-webconsole...